1. Field of the Invention
The present invention relates to a high Mn content nonmagnetic alloy steel which is high in machinability.
2. Description of the Prior Art
Nonmagnetic alloy steels have been extensively used for component parts in electromagnetic apparatus. Typical of such nonmagnetic alloy steels is an austenite stainless steel. This steel usually contains more than 8% Ni by weight and is therefore relatively expensive and low in yield strength. As a consequence, an austenite stainless steel can be only used for component parts which do not require high mechanical strength and for small electromagnetic parts.
Nonmagnetic alloy steels have been recently used for such large scale constructions as magnetically levitated high speed ground transportation facilities (linear motor cars) and nuclear fusion reactor equipment. A very high magnetic field is associated with such equipment. When the structural steel and concrete reinforcing steel bars of such equipment are magnetized in this magnetic field, various undesirable effects will be produced. Furthermore, in terrestial magnetism observing facilities, fine magnetic changes must be precisely measured. The steel to be used for constructing such facilities is required to be high in strength and to have nonmagnetic properties. The nonmagnetic steel for such uses is utilized in comparatively large quanities and must therefore be inexpensive and yet should have the required mechanical strength.
In addition, nonmagnetic steel may have to be partly screw-threaded for use in pre-stressed concrete bars (PC-bars) or may have to be machined for use as component parts such as bolts and nuts. Therefore the steel must be high in machinability.
Examples of known nonmagnetic alloy steel having Mn as a main alloying element, being low in price and having high mechanical strength are disclosed in Japanese Laid-Open Pat. Nos. 150,721/52 and 150,720/52/1976 by one of the present inventors. However, in the disclosed steels, the difficulties in machinability of nonmagnetic alloy steel have not been solved.
On the other hand, a nonmagnetic alloy steel having improved machinability is known from U.S. Pat. No. 4,009,025. This steel has an Ni content of 3.5 to 5.5% by weight which is lower than the Ni content of the conventional austenite stainless steel and therefore the steel is comparatively inexpensive but is still relatively high in price because it contains significant amounts of Ni. Consequently, it is not economical to use this steel in large quantities for the above described applications.